Abstract: Classifies and analyzes the electromechanical coupling system of hybrid electric vehicles. Based on their mathematical model, conditions of coupling are set up, rules of classification are built, and electromechanical coupling systems are classified into three basic categories, viz., torque coupling, speed coupling and power coupling. Several representative electromechanical coupling systems are analyzed. The three basic kinds of system and dual-mode coupling system are evaluated. Power coupling system and dual-mode coupling system are seen to have great fuel economy potential and bright prospect.